Roof venting, ridge vent or power fan?

Feel free too - but you are disagreeing with established government
regulations[1] and not my personal opinion (OK, it's the UK, but the physics
and aim of preventing condensation on timbers remains the same).
25mm or 1" is obviously an arbitrary rounding for convenience, but they will
not be far out from whatever was worked out as "ideal".
[1] This branch of regulation is actually quite sane and deviating from it
would require actually thinking "why do I think i know better?".

From the International Code Council:
---begin quote
R806.1 Ventilation required.
Enclosed attics and enclosed rafter spaces formed where ceilings are applied
directly to the underside of roof rafters shall have cross ventilation for
each separate space by ventilating openings protected against the entrance
of rain or snow. Ventilation openings shall have a least dimension of 1/16
inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Ventilation openings
having a least dimension larger than 1/4 inch (6.4 mm) shall be provided
with corrosion-resistant wire cloth screening, hardware cloth, or similar
material with openings having a least dimension of 1/16 inch (1.6 mm)
minimum and 1/4 inch (6.4 mm) maximum. Openings in roof framing members
shall conform to the requirements of Section R802.7. Required ventilation
openings shall open directly to the outside air.
R806.2 Minimum vent area.
The minimum net free ventilating area shall be 1/150 of the area of the
vented space.
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm
--- end quote
Evidently, reliance on the UK "code" (not cited) is a recipe for disaster.

The source he cited is used as the basis for the code in
much of the USA, isn't it? From everything I've seen, that 1:150
ratio is widely used by building science experts as what is
recommended today. You can probably get away with 1:300,
which was the older standard, if you have to. The reasons
for requiring adequate ventilation are well known.

Not sure. I've used the UBC and SBC years ago and there's another
called BOCA. Don't recall using that one. Here's a reference for
you but I would still call the local jurisdiction to see what they use
in your county. I agree with all your research, you probably are
correct but as long as you are going to this much trouble might as
well do it all the way.
http://www.starrco.com/misc_nbc_codes.php
Keep us posted when/what you decide <grin>.

Did you see my post? Your math had one substantial error
and one whopping 12X error. Applying the 1:150 ratio, you claimed
your 25x25ft attic needed 20 sq ft. It only needs 16.6. A bigger
mistake
was when you came up with 1400 linear FEET. It's not feet, it's
INCHES. Doing the math correctly, that attic would need 200 sq in
of venting. Further, that is TOTAL venting, ie it's split between
intake at the soffits, exhaust at the top. So, to meet the 1:150
ratio, you only need 100 sq in at the soffits. With a 25x25 attic,
that would be met by 1" wide openings run along the two
sides of the roof, 2x25x10.
What Tim is saying is done in the UK would easily exceed that, if
it's done on all 4 sides, with 1". Or using 2" on just the two sides
would do it too. Curiously, that is what I have here in the USA.
Also, I see your reference to the current Intrnational Code being
1:150.
I agree that's sounds like a desirable number and it's recommended.
I think it's a good idea for a number of reasons.
However the roofing manufacturers who are giving a very long, if
not lifetime warranty on their products, say 1:300 is the acceptable
minimum. At least the ones I've looked at in detail.

t

're

text -

I agree erring on the side of having more is better than having less.
But going nuts and putting in 10x what is needed doesn't make any
sense to me either.

You might want to recheck that math. At the ratio of 1 sq ft for
every 150 sq ft of attic floor, you need 16.7 sq ft, not
20. That is 2400 sq in. With a 2" wide vent, you'd need
1200 linear INCHES, not feet. In feet it's 100 feet.
But there is another thing missing here. That ratio of
1 sq ft of venting for every 150ft of attic floor is for TOTAL
venting, which is supposed to be divided between intake
and exit. So, for the example, you'd need only 50 ft of
soffit venting. Using soffit venting on both halves of the
roof, you'd have twice that. I was making this mistake
earlier today when I was doing the math too.
And the minimum acceptable venting ratio is 1 sq ft for
300 sq ft of attic floor, so I should have plenty.

at

If anything, you probably need more with big fans. One
big issue with attic fans, and why most experts are against
them is that they create negative pressure in the house.
This winds up sucking air conditioned air through any
available route, eg ceiling recessed lights, up into the attic.
So, you're increasing the cooling load on the house.

One unasked for thought...
If you have pull down attic stair or even just an access panel, built one
of these to insulate the opening:

http://m.youtube.com/#/watch?v=8POZvK9QB6k

I just recently built one and realized I should have done it 25 years ago
when I first moved in. When I pull down the attic stairs and put my head up
into the box, I can feel the heat that the box is keeping from going up
into the attic.
When I had my roof replaced last year, I had full soffit vents cut and a
ridge vent installed. I also added baffles to improve the air flow. No
icicles on my house when other houses had them. Snow remaining on my roof
after other houses had none.
Prior to the new roof, I had no soffit vents and only gable end vents.
Icicles every year, ice dams kept at bay with heat wires. This year, no
wires, no icicles. I'm pleased.

So, the vents let out the heat, instead of your heat staying in? That could
be a bad thing. You have insullation to keep the heat in while the roof is
cold?
Christopher A. Young
Learn more about Jesus
www.lds.org
.
wrote in message
When I had my roof replaced last year, I had full soffit vents cut and a
ridge vent installed. I also added baffles to improve the air flow. No
icicles on my house when other houses had them. Snow remaining on my roof
after other houses had none.
Prior to the new roof, I had no soffit vents and only gable end vents.
Icicles every year, ice dams kept at bay with heat wires. This year, no
wires, no icicles. I'm pleased.

The soffit vents, ridge vents and soffit baffles keep the roof surface cool
to prevent the snow from melting down to the cold overhang and freezing,
causing ice dams. That's one of the reasons for adequate attic
ventilation.

If you have no insullation in your attic, that could mean more heat used. If
you hvae insullation between you and the vents, then the vents are a good
thing.
Christopher A. Young
Learn more about Jesus
www.lds.org
.
wrote:

The soffit vents, ridge vents and soffit baffles keep the roof surface cool
to prevent the snow from melting down to the cold overhang and freezing,
causing ice dams. That's one of the reasons for adequate attic
ventilation.

Well, I guess I just assumed that we were discussing a typical house
with at least some attic ventilation.
I don't think too many people would pay to have soffit vents, ridge
vents and baffles installed if that meant the heat would just fly out
through the roof.
On the other hand, keeping the heat in (assuming there is no
insulation) - especially in the summer - would mean that you were
cooking the roof from underneath.
Bottom line: Insulation and ventilation are both required for energy
efficiency, comfort and roof longevity.
Here's what I know: The new venting and baffles have significantly
improved how the snow acts on the roof. The attic stair cover I
mentioned in another post is limiting the amount of heat lost via the
stairway access opening. All in all, I'm happy with the upgrades.

The ideal is to seal the cold side against vapour transfer and heat loss and
ventilate sufficiently.
The 25mm-continous figure for soffit venting in the UK comes from BS5250
http://www.commercialconnections.co.uk/Content/building_standards/BS5250%20-%20Code%20of%20Practise%20for%20condensation%20in%20buildings.pdf
which is referred to by Approved Document C (Building Regulations):
http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_C_2010.pdf
The 25mm is conservative - it does say it is not always required, but
builders tend to install by default, unless you have a breathable roof
membrane under failry air-leaky tiles.
In my case, the roof itself is seal almost completely airtight with PIR foam
boards. Once I have an additional vapour barrier under that to reduce
moisture movement into the rafters (which bridge the hot and cold sides of
the insulation, therefore are subject to the risk of interstitial
condensation) I will need very little ventilation into the batten space.
If I still had bitumen felt sarking under the tiles, I would need the 25mm
vent in the eaves and ridge vents.
A lot of older houses here insulate the attic floor and a naturally leaky
roof allows enough ventilation. However, people have come unstuck when
they've increased the insulation and rammed loads of glasswool onto the
eaves blocking the gaps. Coupled with very moisture permeable ceilings leads
to disasterous amounts of condensation in the attic in colder months as some
have found to their cost.

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